The reworking of composite structures frequently requires the localized application of heat. When installing a patch in a rework area of a composite structure, heat must typically be applied to the adhesive at the bondline between the patch and rework area in order to fully cure the adhesive. When applying heat to the patch, the temperature of the bondline must typically be maintained within a temperature range that must be held for an extended period of time until the adhesive is cured. Overheating or under heating the rework area or structure located adjacent to the rework area is generally undesirable during the rework process.
Conventional heating equipment for heating composite structures may include heating blankets comprised of electrically resistive heating elements. Variations in the construction of conventional heating blankets may result in differential heating across the rework area. In addition, conventional heating blankets may lack the ability to compensate for heat sinks located adjacent to the rework area. Such heat sinks may comprise various elements such as stiffeners, stringers, ribs, bulkheads, and other structural members in thermal contact with the structure. Attempts to provide uniform heat distribution using conventional resistive heating blankets include multi-zone blanket systems, feedback loop systems, positive temperature coefficient heating elements, and temperature stabilizing plugs. Additions of such systems to conventional resistive heating blankets are generally ineffective in providing a substantially uniform temperature without substantial variation across the bondline of the rework area.
As can be seen, there exists a need for a system and method for heating a structure such as a rework area of a composite structure in a manner which maintains a substantially uniform temperature across the rework area. More specifically, there exists a need for a system and method for uniformly heating a composite structure and which accommodates heat drawn from the rework area by heat sinks and other thermal variations located adjacent to the rework area. Furthermore, there exists a need for a system and method for uniformly heating a composite structure in a manner which prevents overheating or under heating of the composite structure. Ideally, such system and method for uniformly heating the composite structure is low in cost and simple in construction. There is also a need for a system that provides for temperature regulation over a broad range of temperatures typically required for composite processing, for example, from about 150° F. to about 350° F.
There is also a need for a system that reduces certain unwanted induction effects that may be generated by high frequency electric currents in nearby conductive material, such as metal tooling and graphite composite structures.